Development of cryogenic detectors for rare event searches

The experimental evidence of neutrino oscillations clearly showed that the neutrino is a finite-mass particle. Anyway, two big questions concerning the neutrino are still unsolved: its nature (Dirac or Majorana) and the absolute value of its mass. The Double Beta Decay without emission of neutrinos (0nDBD) is at present the most sensitive method to answer the two questions. Bolometers, together with germanium diodes, have provided so far the best results within this kind of research. The choice of the so-called calorimetric approach, where the detector is made of the 0nDBD material, allows the study of large quantities of isotope with an excellent energy resolution (around 0.2-0.3 %). Both features are crucial to disentangle the searched peak from background. The Cuoricino experiment, in which 62 detectors of tellurium dioxide (TeO2) were involved, was carried out in hall A at Laboratori Nazionali del Gran Sasso (LNGS), and represented the bolometric experiment with the best sensitivity concerning the study on the 0nDBD decay. The knowledge achieved in the bolometric technique and its excellent results on 0nDBD led to the development of new experiments to study rare events with bolometric technique. The CUORE (Cryogenic Underground Observatory for Rare Events) experiment, composed by 988 bolometric detectors of TeO2, is under construction and foresees a sensitivity on neutrino mass of the order of about 50 meV. This high sensitivity requires excellent energetic resolutions, low number of spurious counts within the region of interest and high quantity of the 0nDBD active isotope. The reduction of the background in the energetic window where the 0nDBD for the isotope 130Te is expected (Q_bb = 2527 keV) plays a primary role. In this context, the activities I have done are focused primarily on the analysis of the different contributions to the background of CUORE and the optimization of methods for its reduction. This work has been done both through measurements performed in the hall C of LNGS and with Monte Carlo simulations that allowed data interpretation and assessment of the resulting sensitivity. For example, through a series of simulations performed with Geant4 code it is possible to extrapolate the background induced by muons in CUORE. The results of simulations were then validated through the installation of a muon veto above Cuoricino which allowed a direct comparison with experimental measurement. For what concerns the reduction of the bolometers specific background I have also done some measurements for the development, characterization and optimization of some scintillating bolometers. These detectors, thanks to double read-out of thermal and scintillation signals, allow to discriminate different ionizing particles (beta/gamma, alpha and neutrons). This allows a significant reduction of the unwanted background in the region of interest and would therefore allow to approach the condition of `zero background' experiments. Indeed, thanks to the wide choice for the absorber material that allows to study practically all 0nDBD candidate isotopes, it is possible to choose an isotope with a transition energy above 2615 keV and then remove, in practice, all the background due to gamma rays. The 2615 keV gamma line corresponds in fact to the highest energy gamma-ray line from natural radioactivity and is due to 208Tl. Above this energy there are only extremely rare high energy gamma's. Once gamma-rays are no more a worrisome source of background, what is left on the side of radioactivity are alpha emissions. Indeed alpha surface contaminations are already recognized as the most relevant background source in the bolometric experiment Cuoricino. However, thanks to the double read-out of scintillating bolometers, this source of background can be removed too. During my PhD I have tested a number of different crystals and with some of them (CdWO4, ZnSe and some molybdates) I have obtained excellent results both from the point of view of the detector performance and the reduction of the background contributions. Thanks to these results it was possible to evaluate the discrimination power (i.e. the capability to recognize and reject unwanted events) of this technique and extrapolate the sensitivity of a large mass experiment for 0nDBD based on the hybrid heat plus scintillation technique. Moreover, during the analysis of the data collected with the scintillating bolometers, I recognized for the first time, a dependence of the pulse shapes (both on the scintillation and heat channels) on the interacting particle nature. I proposed then to exploit such wonderful feature to discriminate the interacting particles without relying on the much more complicated measurement of both (light and heat) signals. This feature is very promising because it allows to greatly reduce the background in bolometers without getting complicated the assembly of the experiment. In fact in the case of the double read-out of temperature and scintillation both light detectors and reflecting sheet (used to properly collect the scintillation light) are needed in addition to the low temperature calorimeter. Finally, I have proposed a further use of the scintillating bolometer for diagnostic purposes, i.e. the possibility to study surface contaminations with high sensitivity. One of the main limitations in our understanding of the background due to surface contaminations is in fact related to the limited sensitivity of the available standard techniques. Traditionally the devices used in this field are Si surface barrier detectors with an active area of about 10 cm^2, a typical energy resolution of about 25-30 keV FWHM, and counting rates of the order 0.05 count/h/cm^2 between 3 and 8 MeV. A Cuoricino-like bolometer can easily reach a much larger active area (150 cm^2) and, thanks to the absence of a dead layer can reach resolution on surface alpha particles of 10 keV. Moreover a background counting rate in the 3-8 MeV region as low as 0.001 count/h/cm^2 was already reached with this technique. This considerations allow to plan measurements with sensitivities order of magnitude better than standard devices. However, in order to use scintillating bolometer to study surface contaminations, they have not to be surrounded by a reflecting sheet. For these reason it is necessary to use crystals with a very high light yield or crystals that are able to recognize particle from the shape of the thermal pulses (i.e. without any need of collecting the scintillation light). Since this last feature is a very recent discovery and some works have still to be performed before the technique can be considered actually at hand, I have proposed to use a BGO crystal (Bi4Ge3O12), which is characterized by a very high light yield, to study surface contaminations. Preliminary tests with an array of 4 `small' crystals (2x2x2 cm^3) have shown how this possibility could be fulfilled. However this measurement showed a very slow cooling down and an high counting rate due to 207Bi. Therefore, before using the crystal to study surface contamination, a new measurement was carried out with a larger crystal (5x5x5 cm^3). The measurement was successful and showed that the slow cool down and the high counting rate are not intrinsic problems of these crystals. This first test gave excellent results on surface studies and, as supplementary results, a measurement of the rare alpha decay of 209Bi with a high statistical significance was performed.